Certain hydrocarbon mixtures are difficult to separate by ordinary distillation, therefore, extractive distillation may be used to effect such separation. Extractive distillation is extensively used to separate hydrocarbons with like numbers of carbon atoms but with different degrees of unsaturation. Extractive distillation involves distilling the mixture to be separated in the presence of a relatively higher boiling solvent which is selective for at least one component of the mixture. An extractive distillation process is normally performed by introducing the selective solvent into a distillation column at a point near the top of the distillation column and allowing the solvent to flow down the distillation column as the distillation proceeds. As the hydrocarbon vapors travel up the distillation column, they are contacted by the solvent and the more highly unsaturated hydrocarbons are caused to have a reduced relative vapor pressure compared to the other unsaturated hydrocarbons. The bottoms of the distillation column then consists primarily of the solvent plus the more highly unsaturated hydrocarbon. The more highly unsaturated hydrocarbon may be removed from the solvent in a stripping column or by other suitable means and the lean solvent is recycled to the extractive distillation column.
Extractive distillation is especially useful in separating mixtures of C.sub.4 or C.sub.5 hydrocarbons with different degrees of unsaturation. The C.sub.4 and C.sub.5 hydrocarbons are normally obtained from catalytic or thermal cracking, dehydrogenation, including oxidative dehydrogenation, or Fisher-Tropsch reactions and the like. Isoprene is a particularly desirable C.sub.5 hydrocarbon.
The C.sub.5 feed stock used in the production of isoprene contains isoprene and 2-methyl-2-butene along with other hydrocarbons. In the production of isoprene, it is necessary to separate isoprene from 2-methyl-2-butene. Since isoprene is the more unsaturated of the two C.sub.5 hydrocarbons, an extractive distillation solvent is employed which will selectively extract the isoprene vapors and collect in the bottoms while allowing the 2-methyl-2-butene vapors to pass upward and be withdrawn from the top of the extractive distillation column. Isoprene and the solvent is withdrawn from the bottom and passed to a stripper where the isoprene is separated from the fat solvent and the lean solvent is then returned to the distillation column.
In the extractive distillation process of separating isoprene from 2-methyl-2-butene, several solvents are proposed in the art, including furfural, acetonitrile, dimethylformamide, N-methylpyrrolidone. Small amounts of water are normally used with these solvents. Water is not a cosolvent but serves to improve the selectivity of the solvent and to lower the temperature in the stripping process.
Unfortunately, many of the solvents proposed in the art are either relatively poor in selectively, corrosive, or result in equipment fouling.
U.S. Pat. No. 3,898,135 discloses an extractive distillation solvent comprising a mixture of 3-methoxypropionitrile with small amounts of water and furfural. This solvent mixture improves the separation process of C.sub.4 hydrocarbons by reducing fouling in the distillation equipment.
U.S. Pat. No. 3,890,208 discloses an extractive distillation cosolvent comprising a mixture of furfural with small amounts of water an an acyclic ketone. This cosolvent has improved selectivity in separating n-butane and butene-1 as compared to a solvent of furfural and water.
Acetonitrile with small amounts of water has been found to be a prticularly desirable extractive distillation solvent in the separation of isoprene from 2-methyl-2-butene because of its lack of corrosion and fouling of the equipment and also because of the cost and availability of acetonitrile. U.S. Pat. No. 3,230,157 discloses a process for producing high purity isoprene wherein acetonitrile with up to 5 weight percent of water is used as the extractive distillation solvent.
However, it is desirable to improve the efficiency of an extractive distillation process employing acetonitrile as the solvent for separating isoprene from 2-methyl-2-butene.